When the movable element of certain control devices is moved from a stationary position by an actuator, static friction must typically be overcome before the control element can begin to move. For controlling a control device having an electric actuator such as a linear or rotary electric motor that moves a control valve element, known control strategies can provide an electrical solution for adjustment of the control signal to the actuator to provide the increased force or torque needed to overcome static friction. However, once static friction has been overcome, the added force or torque typically becomes unnecessary, and indeed often undesirable.
When an actuator, or some portion of the load that is moved by an actuator, includes a biasing member such as a return spring, it may be desirable to include compensation for the variable force or torque exerted by such as biasing member as part of the overall control strategy.
It is also known to incorporate a variable ratio drive mechanism as a mechanical solution for compensating for opposing force or torque that changes in some way either linearly or non-linearly as a function of the position and/or velocity of a load that is being moved by an actuator, such as when a return spring is present. The function of a variable ratio drive mechanism is to provide an improved torque/force advantage over a particular region or regions of motion while providing reasonable response or speed of movement over the complete range of motion. Such a mechanical solution may be used by itself or in conjunction with an electrical solution.
A gear-type variable ratio drive mechanism is one type of such a mechanism. Incorporation of this type of mechanism into an actuator involves gear ratio selection. For the capability of a particular electric actuator to move a load, the gear ratio that is finally selected is inherently a compromise between adequate torque/force and speed of motion because increasing the ratio to deliver more force/torque to the load reduces the speed of movement of the load, and vice versa.
Furthermore, for any of various reasons other than static friction and biasing, reasons that may depend on the particular type of control device being operated, the effective loading on the actuator may be significantly different over different portions of the range of motion of the movable element. For example, sticking due to contamination or a change in differential fluid pressure acting on a moveable valve element, such as when the valve element is cracked open, can change the load imposed on the actuator in a way that calls for some sort of compensation, either electrically and/or mechanically.
When varying force or torque requirements have to be compensated in the presence of cost and/or environmental and/or packaging constraints, optimal solutions can be difficult to realize.